Methods for forming a thin film by means of gas phase deposition can be broadly divided into chemical vapor deposition (CVD) and physical vapor deposition (PVD).
PVD typically includes vacuum vapor deposition and sputter deposition. In particular, sputter deposition enables production of a high quality thin film with uniform film properties and film thickness, although the apparatus cost is usually high. For this reason, sputter deposition is widely applied to display devices and the like. However, the film may have defects.
CVD is a process of growing a solid thin film by introducing a raw material gas into a vacuum chamber, and decomposing or reacting one type or two or more types of gas on a substrate by means of thermal energy. In some CVD processes, a plasma or catalyst reaction may be used in combination to promote the reaction or decrease reaction temperature. CVD using plasma is called plasma enhanced CVD (PECVD), and CVD using a catalyst reaction is called catalytic CVD (Cat-CVD). Although chemical vapor deposition is characterized by having fewer defects in film formation and is mainly applied to production of semiconductor devices such as film formation of a gate insulation film, it has a disadvantage in that a relatively high temperature is required for film formation.
Atomic layer deposition (ALD), which is classified as CVD, is a film formation process in which a film is formed in a layer-by-layer manner at an atomic level by chemically reacting the substances which are adsorbed on the surface. ALD is distinguished from general CVD in that general CVD uses a single gas or concurrently uses a plurality of gases which are reacted on a substrate to thereby grow a thin film on a substrate, while ALD is a particular film formation method which uses a highly reactive gas, which is referred to as a precursor, and a reactive gas (which is also referred to as a precursor) to perform adsorption on the substrate surface and subsequent chemical reactions to thereby grow a thin film in a layer-by-layer manner at an atomic level.
Specifically, ALD uses a self-limiting effect in surface adsorption that prohibits a certain type of gas from being adsorbed onto a surface after the surface is covered by the gas. Accordingly, after one layer of precursor is adsorbed onto the surface, unreacted precursor is purged. Then, another reactive gas is introduced to oxidize or reduce the above precursor to thereby obtain one layer of a thin film having a desired composition. After that, the reactive gas is purged. This cycle is repeated so as to grow a thin film in a layer-by-layer manner. Accordingly, the thin layers grow in two dimensions in the ALD process. ALD is characterized by reducing deficiencies in film formation compared with typical CVD as well as with conventional vapor deposition or sputtering, and is expected to be applied to various fields.
ALD may include a process of using plasma to enhance the reaction in the step of decomposing a second precursor and reacting the decomposed second precursor with a first precursor adsorbed on the substrate, which is called plasma enhanced ALD (PEALD) or simply plasma ALD.
Since ALD is characterized by having no projection effect or the like compared with other film formation methods, film formation only require a gap through which a gas is introduced. Accordingly, ALD is expected to be applied to fields related to micro electro mechanical systems (MEMS) for covering a three-dimensional structure, as well as for covering lines and holes having a high aspect ratio.
By using the aforementioned film formation method, a thin film is formed on a variety of targets including a small plate-shaped substrate such as a wafer or photomask, a substrate having a large surface area and no flexibility such as a glass plate, or a substrate having a large surface area and flexibility such as a film, and the like. Accordingly, for mass production facilities that form a thin film on these substrates, a variety of handling methods for the substrates which are different in cost, ease of handling, film formation quality, and the like have been proposed and put to practical use.
For example, there are sheet type deposition, batch type deposition, and the like. In sheet type deposition, film formation is performed while one sheet of substrate is supplied on a wafer in a film forming apparatus, and then, film formation is again performed after the sheet is replaced with a subsequent substrate. In batch type deposition, a plurality of substrates are collectively set so that the same film formation is performed onto all the wafers.
Further, methods for forming a film on a glass substrate or the like include an in-line method, in which film formation is concurrently performed while a substrate is successively transported to a portion of a deposition source, and a web coating method using a so-called roll-to-roll method, in which film formation is performed mainly on a flexible substrate while the substrate is paid out from a roll and transported, and then the substrate is taken up by another roll. The web-coating method also includes a transportation/continuous film formation process, in which a substrate on which a film is formed, as well as the flexible substrate, is continuously transported on a flexible sheet or on a partially flexible tray.
An improved combination is adopted from the above film formation methods and the handling methods for the substrates, considering the cost, quality, ease of handling, and the like.
ALD has disadvantages such as use of specific materials, and its cost. Among others, since ALD is a process which grows a thin film at an atomic level by depositing a layer in each cycle, the most significant disadvantage is that the film formation speed is 5 to 10 times slower than other film formation methods such as vapor deposition and sputtering.
To solve the above problem, contrary to the conventional process in which a precursor is repeatedly supplied and discharged in a single chamber (which is called a time-divided type), a space-divided type has been proposed, in which the chamber is divided into several zones so that a single precursor or purge gas is supplied into the respective zones while the substrate is reciprocated among the zones (for example, see PTL 1).